Optical scanning device and image forming apparatus using the same

ABSTRACT

There is provided a diffraction grid which is easily produced without reducing a grid size. There is provided an optical scanning device having effects such as a chromatic aberration correction and a temperature compensation even when a short wavelength light source having a wavelength of 500 nm or less is used, and an image forming apparatus using the optical scanning device. In the optical scanning device having the diffraction grid, for which the short wavelength light source having a wavelength of 500 nm or less is used, a design order of the diffraction grid is set to a diffraction order equal to or larger than a second order to obtain a grid shape which is easily formed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical scanning device and an imageforming apparatus using the same. In particular, the present inventionrelates to an optical scanning device that is suitably used for anapparatus such as a laser beam printer or a digital copying machinehaving an electrophotographic process, in which a light flux opticallymodulated and emitted from light source means is reflected and deflectedon a polygon mirror serving as optical deflection means and then asurface to be scanned is scanned with the light flux through a scanningoptical system to record image information. The present inventionrelates to an optical scanning device capable of providing asatisfactory image by adopting a diffraction grid for correction ofchromatic aberration of magnification or temperature compensation and toan image forming apparatus using the same. In addition, the presentinvention relates to a color image forming apparatus which uses aplurality of optical scanning devices and is composed of a plurality ofimage bearing members corresponding to respective colors.

2. Related Background Art

Up to now, in an optical scanning device used for a laser beam printer(LBP) or the like, a light flux optically modulated according to animage signal and emitted from light source means is periodicallydeflected by, for example, an optical deflector composed of a rotatingpolygonal mirror (polygon mirror). The deflected light flux is convergedto form a spot shape on a photosensitive recording medium(photosensitive drum) by a scanning optical system having an fθcharacteristic. The surface of the recording medium is scanned with thelight flux to perform image recording.

FIG. 13 is a schematic view showing a main part of a conventionaloptical scanning device.

In FIG. 13, a divergent light flux emitted from a light source means 1is converted into a substantially parallel light flux by a collimatorlens 3. The substantially parallel light flux is limited by a diaphragm2 and incident into a cylindrical lens 4 having predetermined refractivepower only in the sub scanning direction. Of the substantially parallellight fluxes incident into the cylindrical lens 4, the light flux withina main scanning section outgoes therefrom without changing an opticalstate. The light flux within a sub scanning section is converged andimaged as a substantial linear image onto a deflection surface(reflection surface) 5 a of a deflecting means 5 composed of a polygonmirror.

The light flux which is deflected on the deflection surface 5 a of thedeflecting means 5 is guided onto a photosensitive drum surface 8serving as a surface to be scanned through a scanning optical system 6having an fθ characteristic. By rotating the deflecting means 5 in adirection indicated by an arrow “A”, the photosensitive drum surface 8is scanned with the light flux in a direction indicated by an arrow “B”to record image information.

Further, in order to achieve high speed scanning, a multi-beam opticalscanning device that simultaneously forms a plurality of scanning linesby light fluxes from a plurality of light sources has been proposed andcommercially available from various companies. FIG. 14 is a schematicview showing a main part of a multi-beam optical scanning device. Twolight fluxes emitted from light sources 81 and 82 are converted intoparallel light fluxes by collimator lenses 83 and 84 and thensynthesized into one by a synthesizing optical element 85. Thesynthesized light flux forms a linear image extended in the mainscanning direction near a deflection surface of a polygon mirror 87 bythe action of a cylindrical lens 86 and then forms a light spot on aphotosensitive drum 89 by a scanning optical system 88. Therefore, thetwo scanning lines can be formed by performing optical scanning once, sothat extremely high speed scanning can be achieved as compared with aconventional optical scanning device. With respect to a multi-beam lightsource other than one using the above-mentioned synthesizing opticalelement, a monolithic multi-beam laser in which a large number of lightemitting points exist has been produced. In the case where themonolithic multi-beam laser is used, it is unnecessary to use thesynthesizing optical element. Thus, it is possible to simplify theoptical system and the optical adjustment.

In an optical scanning device using a multi-beam light source, in orderto eliminate a jitter caused by a wavelength difference between aplurality of light sources (variation in interval between scanning lineson the photosensitive drum surface in the main scanning direction), anycountermeasure such as the appropriate selection of the light sourceshas been taken so as to minimize the wavelength difference between thelight sources. When the jitter caused by the wavelength differencebetween the light sources (chromatic aberration of magnification) iscorrected by the scanning optical system, a plurality of lenses havingdifferent dispersion characteristics are required. As compared with ascanning optical system that does not correct the chromatic aberrationof magnification, the number of lenses generally increases to cause anincrease in cost. There is a limitation with respect to the range ofselection of the wavelengths of the light sources. Therefore, it isdifficult that the wavelengths are made completely equal to one another.There is a problem with respect to a cost for the selection of thewavelengths. When a semiconductor laser is activated, an image qualityis deteriorated by a variation in wavelength, which is called modehopping. Thus, even in an optical scanning device other than the opticalscanning device using the multi-beam light source, in order to improvethe stability of the image quality, it is necessary to minimize thejitter caused by the variation in wavelength.

A semiconductor laser used as a conventional light source (as disclosedin, for example, Japanese Patent Application Laid-Open No. H10-197820and Japanese Patent Application Laid-Open No. H10-068903) is an infraredlaser (780 nm) or a visible laser (675 nm). However, in order to realizea high resolution, the development of an optical scanning device inwhich a minute spot shape is obtained by using a short wavelength laserhaving an oscillating wavelength of 500 nm or less is under way. Theadvantage of the use of the short wavelength laser is that a minute spotsize which is about half of a conventional spot size can be achievedwhile an exit F number of the scanning optical system is kept equal to aconventional one. In the case where a spot size is reduced to half ofthe conventional spot size while using the infrared laser, it isnecessary to increase the intensity of the scanning optical system to anintensity about two times larger than that in a conventional case. Afocal depth is proportional to a wavelength of a used light source andto the square of the exit F number of the scanning optical system.Therefore, to obtain the same spot size, the focal depth in the infraredlaser becomes equal to or smaller than about ½ of the focal depth in theshort wavelength laser.

In such an optical scanning device, in order to record image informationwith high precision, it is necessary to preferably correct a curvatureof field over the entire surface to be scanned, to have a distortioncharacteristic (fθ characteristic) with a constant speed, between anangle of view θ and an image height Y, and to make spot sizes on theimage plane uniform at respective image heights. Various opticalscanning devices or various scanning optical systems that satisfy theoptical characteristics like those have been proposed up to now.

According to Japanese Patent Application Laid-Open No. H10-197820 andJapanese Patent Application Laid-Open No. H10-068903 as described above,a (temperature compensation) optical scanning device using a diffractionoptical element for a scanning optical system has been proposed toreduce a focal variation on a surface to be scanned due to thecorrection of the chromatic aberration of magnification and anenvironmental variation.

In particular, in an optical scanning device using a short wavelengthlight source having a wavelength of 500 nm or less, a dispersioncharacteristic of a material used for a scanning lens is large.Therefore, the chromatic aberration of magnification becomes six timesor seven times larger than that in a conventional infrared laser. Thus,in the optical scanning device using the multi-beam laser, the jitter issignificantly caused in the main scanning direction to reduce the imagequality.

When a blue-violet laser which is made of a material such as galliumnitride and oscillated at the wavelength of 405 nm is used to obtain aspot size which is about half of a spot size of the infrared laser, asdescribed above, the focal depth is proportional to the wavelength.Therefore, only a depth about half the conventional depth can beallowed. Thus, along with the improvement of precision of respectiveparts composing the optical scanning device, a (temperaturecompensation) optical scanning device in which the focal variation isnot caused even in the case of the environmental variation is desired.

As described below, a phase function φ for determining a grid shape isinversely proportional to the wavelength. Therefore, when a diffractiongrid having the same power is designed, a grid pitch of the shortwavelength laser having 500 nm or less becomes smaller than that of theconventional infrared laser.

For the above-mentioned reason, when a light flux having a shortwavelength of 500 nm or less is imaged on the surface to be scanned inthe optical scanning device using the short wavelength light sourcehaving a wavelength of 500 nm or less, there is a problem in that a gridsize becomes smaller.

In general, as shown in FIG. 15A, a grid shape formed on a mold by moldforming is transferred to an optical resin member or an optical glassmember. After that, as shown in FIG. 15B, the transferred grid shape isseparated from the mold to produce the diffraction optical element.Alternatively, a method of dropping a small amount of ultravioletcurable resin on a glass substrate (lens can be also used) serving as abase and curing the grid shape similarly formed on the mold byultraviolet light is used.

However, as the grid size becomes smaller, the following problems arecaused:

it is hard to produce the mold by a cutting tool;

transfer property of the grid shape formed on the mold deteriorates; and

along with the deterioration of the transfer property, an imagingperformance deteriorates and a diffraction efficiency reduces.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a diffraction gridwhich can be used for an optical scanning device and easily produced byselecting a suitable design order of the diffraction grid. Anotherobject of the present invention is to provide a diffraction grid thatattains a preferable performance even when a short wavelength lightsource having a wavelength of 500 nm or less is used. Still anotherobject of the present invention is to provide an optical scanning deviceusing the diffraction grid, which has functions such as a chromaticaberration correction and a temperature compensation, and an imageforming apparatus using the optical scanning device.

According to one aspect of the present invention, an optical scanningdevice includes:

light source means for emitting at least one light flux having awavelength of 500 nm or less;

deflecting means for deflecting the at least one light flux emitted fromthe light source means;

a first optical system for guiding the light flux emitted from the lightsource means to the deflecting means; and

a second optical system for imaging the light flux deflected by thedeflecting means on a surface to be scanned,

in which one of the first optical system and the second optical systemincludes a diffraction grid and a design order of the diffraction gridis set to a diffraction order equal to or larger than a second order.

Also, according to another aspect of the present invention, an opticalscanning device includes:

light source means for emitting at least one light flux having awavelength of 500 nm or less;

deflecting means for deflecting the at least one light flux emitted fromthe light source means;

a first optical system for guiding the light flux emitted from the lightsource means to the deflecting means; and

a second optical system for imaging the light flux deflected by thedeflecting means on a surface to be scanned, in which:

one of the first optical system and the second optical system includes adiffraction grid; and

provided that the total number of grids of the diffraction grid isrepresented by m, an effective range of the diffraction grid isrepresented by L (mm), a minimum grid pitch of the diffraction grid isrepresented by P (μm), and the wavelength of the light flux isrepresented by λ (μm), the following expressions are satisfied:m·L·λ<300000  (1)30<P<λ  (2)

In further aspect of the optical scanning device, provided that adiffraction order of the diffraction grid is represented by k, thewavelength of the light flux is represented by λ (μm), and a refractionindex of a material composing the diffraction grid is represented by n,the following expression may be satisfied:1<k·λ/(n−1)<5  (3)

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a main scanning sectional view showing an optical scanningdevice according to a first embodiment of the present invention;

FIG. 2 shows a diffraction optical element in the first embodiment ofthe present invention;

FIG. 3 is a sectional view showing a diffraction grid at a minimum pitchin the case using secondary diffraction light;

FIG. 4 is a sectional view showing a diffraction grid at a minimum pitchin the case using primary diffraction light;

FIGS. 5A and 5B are explanatory views showing mold processing for thediffraction optical element;

FIG. 6 is a graph showing a chromatic aberration of magnification of ascanning optical system in a main scanning direction in the firstembodiment of the present invention;

FIG. 7 is a graph showing a focal variation of the scanning opticalsystem in a sub scanning direction in the first embodiment of thepresent invention;

FIG. 8 is a sub scanning sectional view showing an optical scanningdevice according to a second embodiment of the present invention;

FIG. 9 is a sectional view showing a part of an optical scanning deviceaccording to a third embodiment of the present invention;

FIGS. 10A and 10B are graphs showing a spherical aberration and anon-axis chromatic aberration in the third embodiment of the presentinvention;

FIG. 11 is a main part schematic diagram showing an image formingapparatus according to the present invention;

FIG. 12 is a main part schematic diagram showing a color image formingapparatus according to the present invention;

FIG. 13 is a perspective view showing a conventional optical scanningdevice;

FIG. 14 is a perspective view showing a conventional optical scanningdevice using a multi-beam light source; and

FIGS. 15A and 15B are explanatory views showing a process for forming adiffraction optical element.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 1 is a main scanning sectional view showing an optical scanningdevice according to a first embodiment of the present invention.

Here, a main scanning direction indicates a direction perpendicular tothe rotational axis of deflecting means. A sub scanning directionindicates a direction parallel to the rotational axis of the deflectingmeans. In addition, a main scanning section indicates a plane which isparallel to the main scanning direction and includes the optical axes ofa deflection optical element (lens) 6 and a diffraction optical element(lens) 7 having a diffraction grid 7 a formed on an exit surface of adiffraction part, which compose a scanning optical system.

A divergent light flux emitted from a single beam semiconductor laser 1serving as light source means is limited by a diaphragm 2 so as toreduce a width of the light flux, thereby obtaining a desirable spotsize. Then, the light flux is converted into a substantially parallellight flux by a collimator lens 3. The substantially parallel light fluxis imaged as a linear image extended in the main scanning direction ontothe vicinity of a deflection surface 5 a of a deflecting means 5described later by a cylindrical lens 4 having predetermined refractivepower in only the sub scanning direction. The deflecting means 5 iscomposed of, for example, a polygon mirror having four surfaces(rotating polygonal mirror) and rotated in a direction indicated by anarrow “A” in FIG. 1 at a constant rate by drive means such as a motor(not shown). With the scanning optical system composed of the deflectionoptical element (lens) 6 and the diffraction optical element (lens) 7having fθ characteristics, the deflection light flux which is reflectedand deflected on the deflecting means 5 is imaged onto a photosensitivedrum surface 8 serving as a surface to be scanned. In addition, a tangleerror of the deflection surface 5 a of the deflecting means 5 iscorrected by the scanning optical systems. At this time, the deflectionlight flux which is reflected and deflected on the deflection surface 5a of the deflecting means 5 is guided onto the photosensitive drumsurface 8 through the deflection optical element (lens) 6 and thediffraction optical element (lens) 7. When the polygon mirror 5 isrotated in a direction indicated by an arrow “A”, the photosensitivedrum surface 8 is optically scanned in a direction indicated by an arrow“B”. Therefore, scanning lines are formed on the photosensitive drumsurface 8, thereby performing image recording.

Here, an optical arrangement and figures in this embodiment are shown inTable 2.

A shape of each generating line of the incident and exit surfaces of thedeflection optical element (lens) 6 and the incident surface of thediffraction optical element (lens) 7 is based on an aspherical shapewhich can be indicated as a function of up to the tenth order. Forexample, in the case where an intersection point between the deflectionoptical element (lens) 6 and the optical axis is set to an origin, theoptical axis direction is set to an X-axis, and an axis perpendicular tothe optical axis within the main scanning section is set to an Y-axis, agenerating line direction corresponding to the main scanning directionis indicated by the expression,$X = {\frac{\frac{Y^{2}}{R}}{1 + \sqrt{1 - {\left( {1 + k} \right)\left( \frac{Y}{R} \right)^{2}}}} + {B\quad 4 \times Y^{4}} + {B\quad 6 \times Y^{6}} + {B\quad 8 \times Y^{8}} + {B\quad 10 \times Y^{10}}}$(where R is a curvature radius of a generating line and K, B4, B6, B8,and B10 are aspherical coefficients).

In addition, a meridian direction corresponding to the sub scanningdirection is indicated by the expression,$S = \frac{\frac{Z^{2}}{{Rs}^{*}}}{1 + \sqrt{1 - \left( \frac{Z}{{Rs}^{*}} \right)^{2}}}$S indicates a shape of a meridian line which includes a normal of thegenerating line at each position in the generating direction and isdefined within a plane perpendicular to the main scanning surface.

Here, a curvature radius in the sub scanning direction (meridian linecurvature radius) Rs* at a position away from the optical axis by Y inthe main scanning direction is indicated by the expression,Rs*=Rs×(1+D2×Y ² +D4×Y ⁴ +D6×Y ⁶ +D8×Y ⁸ +D10×Y ¹⁰)(where Rs is the meridian line curvature radius on the optical axis andD2, D4, D6, D8, and D10 are meridian line change coefficients).

Note that the figure is defined by the above-mentioned expressions inthis embodiment. However, the scope of the present invention is notlimited to this.

In the diffraction optical element 7, the diffraction grid 7 a is formedon the exit surface side of the refraction portion having the fθcharacteristic and the grid shape thereof is defined by the followingphase function. That is, the grid shape is expressed by the followingexpression related to the diffraction surface indicated by a phasefunction of up to a tenth order in the main scanning direction and aphase function of a second order in the sub-scanning direction, which ischanged according to a position in the main scanning direction,φ=2πk/λ{b2Y ² +b4Y ⁴ +b6Y ⁶ +b8Y ⁸ +b10Y ¹⁰+(d0+d1Y+d2Y ² +d3Y ³ +d4Y ⁴+d5Y ⁵ +d6Y ⁶)Z ²}(where φ is a phase function, k is a diffraction order, λ is a designwavelength (405 nm), Y is a height from a lens optical axis, and b2, b4,b6, b8, b10, d0, d1, d2, d3, d4, d5, and d6 each is a phase coefficient,secondary diffraction light being used in this embodiment).

Table 1 shows design parameters of the optical system of the opticalscanning apparatus shown in FIG. 1. The lens 6 is the defection opticalelement and the lens 7 is the diffraction optical element in which thediffraction grid is provided on the deflection base surface. TABLE 1Design data Wavelength, Refractive index Lens 6 Figures Use wavelength λ(nm) 405 First plane Second plane First plane Second plane Lens 7Figures Lens 6 Refractive index n₄₀₅      1.5466198 R −8.74966E+01 −4.71453E+01  R −4.04355E+02 ∞ K 5.71925E−01 −1.04001E+00  K−3.69841E+01 Lens 7 Refractive index n₄₀₅      1.5466198 B4 1.40393E−069.40875E−08 B4  2.07946E−07 B6 1.26075E−09 5.42879E−10 B6 −1.81403E−11B8 −1.16558E−12  4.08844E−13 B8  8.57936E−16 Light beam angle B101.96978E−16 −4.10320E−16  B10 −1.82040E−20 Incident angle to polygon θp−70  r0 1.43515E+02 −3.21543E+01  Diffraction grid, Phase coefficientMaximal exit angle on θe 45 D2u 2.46277E−04 b2 −1.01974E−04 polygon D4u−1.56775E−07  b4  2.09153E−09 Arrangement D6u 7.89931E−11 b6−3.45669E−13 Polygon surface to lens 6 e1 30 D2l 1.86868E04  b8 2.59139E−17 Central thickness of lens 6 e2 11 D4l −1.07713E−07  b10−8.18565E−22 Lens 6 to lens 7 e3 75 D6l 5.27788E−11 d0 −3.37934E−03Central thickness of lens 7 e4  5 Suffios u: Opposite Laser Side d1 4.58488E−07 Lens 7 to surface to be Sk 111  Suffios l: Laser Side d2 8.43600E−08 scanned Polygon axis to surface to L  238.4 Y-Axis Sign +:Laser Side d3 −1.80357E−11 be scanned Effective scanning width W 297  d4−2.74067E−12 d5  0.00000E+00 d6  4.41097E−17

Table 2 shows phase coefficients, the total number of grids m within aneffective range L, and a minimum pitch P of grids. TABLE 2 PrimarySecondary Coefficient Order diffraction light diffraction light Mainscanning phase coefficient & Grid pitch b2 y2 −2.039480E−04−1.01974.E−04 b4 y4  4.183050E−09  2.09153.E−09 b6 y6 −6.913380E−13−3.45669.E−13 b8 y8  5.182780E−17  2.59139.E−17 b10 y10 −1.637130E−21−8.18565.E−22 Total grid number m 5680 2840 Effective width (mm) 216 216Minimum pitch (mm) 0.0089 0.0178 Sub scanning phase coefficient & Gridpitch d0 z2 −6.75868E−03  −3.379340.E−03 d1 z2y 9.16976E−07 4.584880.E−07 d2 z2y2 1.68720E−07  8.436000.E−08 d3 z2y3 −3.60713E−11 −1.803565.E−11 d4 z2y4 −5.48134E−12  −2.740670.E−12 d5 z2y6 8.82193E−17 4.410965.E−17 Total grid number m 150 75 Effective width (mm) 6 6Minimum pitch (mm) 0.0100 0.0201

FIG. 2 is a schematic view showing the diffraction optical element 7, inwhich the diffraction grid 7 a is formed in a saw-tooth shape (blazedtype) in the main scanning direction and the sub scanning direction.

In this embodiment, a short wavelength light source having λ=405 nm isused. In the entire scanning region, it is achieved that a spot diameterin the main scanning direction becomes 27 μm and a spot diameter in thesub scanning direction becomes 35 μm. Here, the spot diameter indicatesa diameter of a region which is obtained by slicing at 1/e2 of a peaklight amount.

FIG. 3 shows a grid shape at a minimum pitch in the case where thediffraction grid is designed using secondary diffraction light in thisembodiment. For comparison with FIG. 3, FIG. 4 shows a grid shape at aminimum pitch in the case where the diffraction grid is designed usingprimary diffraction light. As is apparent from FIG. 4, in the case usingthe conventional primary diffraction light, a grid height becomes lowerthan 1 μm and the minimum pitch P becomes a half of that in the caseusing the secondary diffraction light. As is read from table 2, thetotal number of grids of the diffraction grid m becomes two times largerthan that in the case using the secondary diffraction light.

As described above, the diffraction optical element 7 is designed usingthe secondary diffraction light. Therefore, it is possible to obtain thegrid pitch which is about two times larger than that in the case usingthe primary diffraction light and the total number of grids which isabout a half of that in the case using the primary diffraction light.

In other words, in this embodiment, with respect to a light flux havinga wavelength of 500 nm or less, the diffraction order equal to or largerthan the second order is used for diffraction light for forming an imageon the surface to be scanned.

FIGS. 5A and 5B are explanatory views showing a mold cutting process forthe diffraction optical element 7. With respect to a grid shape around acentral portion of the diffraction optical element 7, because the gridpitch is relatively large, the central portion is cut plural times usingan end portion of a cutting tool which is shaded (FIG. 5A). On the otherhand, because the grid pitch in a peripheral portion of the diffractionoptical element 7 is small, the peripheral portion is cut one time foreach grid to obtain the grids each having an oblique surface (FIG. 5B).Mold processing is always performed on the diffraction optical elementwhile the end portion of the cutting tool is damaged. The cutting toolis gradually worn during cutting. Therefore, a problem that a targetgrid shape cannot be obtained occurs.

As the number of grids and the effective width of the diffractionoptical element become larger, the amount of wear of the cutting toolincreases.

Therefore, in the case where the total number of grids of thediffraction optical element is represented by m, the effective range ofthe diffraction optical element is represented by L (mm), and thewavelength of the light source is represented by λ (μm), whenm·L·λ<300000  (1)is satisfied, a grid shape suitable to the wavelength of the used lightsource can be obtained.

Even when the grid pitch is small, a damage to the cutting tool becomeslarger. Therefore, when the minimum pitch of the diffraction opticalelement is represented by P (μm), it is preferable to satisfy30<P/λ  (2).

In this embodiment, m·L·λ=248443 and P/λ=43.95. Therefore, bothExpression 1 and Expression 2 are satisfied.

When the diffraction order of the diffraction optical element isrepresented by k and a refraction index of a material composing thediffraction optical element is represented by n, a grid depth hnecessary to set a diffraction efficiency to 100% at a use wavelength isexpressed to be h=k·λ/(n−1).

When the grid depth is too shallow, processing precision is hard toimprove. When the grid depth is too deep, the damage to the cutting toolbecomes larger. Therefore, the diffraction order and the material areselected so as to satisfy1 <k·λ/(n−1)<5  (3).Thus, a grid shape suitable to the wavelength of the used light sourcecan be obtained.

In the case of the diffraction grid using the short wavelength lightsource having a wavelength of 500 nm or less, it is desirable that thedesign order of the diffraction grid is the second order to the fifthorder. When the diffraction order is the first order, as describedabove, the total number of grids is too large. When the diffractionorder is equal to or larger than the sixth order, the grid depth becomestoo deep, so that a cutting resistance of the cutting tool increases. Inaddition, when the diffraction grid has the diffraction order equal toor larger than the sixth order, significant influence is caused by avariation in diffraction efficiency due to a variation in grid depth.

In this embodiment, the diffraction order k is 2, λ is 0.405 (μm), and arefraction index n₄₀₅ of an optical resin composing the lens 7 is1.54662. Therefore, k·λ/(n−1) becomes 1.4818 (μm), so that Expression 3is satisfied.

Table 3 shows refraction indexes of optical resins used in the first tothird embodiments. A chromatic aberration of magnification and a focalvariation during an increase in temperature are calculated based onvalues shown in Table 3.

In this embodiment, the second diffraction order is used as the designorder of the diffraction grid. Even when the design order equal to orlarger than the third order is used, a diffraction grid which is easilyproduced without reducing a grid size can be provided. Even when theshort wavelength light source having a wavelength of 500 nm or less isused, it is possible to provide an optical scanning device havingfunctions such as a chromatic aberration correction and a temperaturecompensation, and an image forming apparatus using the optical scanningdevice.

Even in the second and third embodiments described below, the designorder equal to or larger than the third order may be used as the designorder of the diffraction grid. TABLE 3 Wavelength (nm) 405 406 410Refractive During normal 1.5466198 1.5464526 1.5457982 index temperature(t = 25° C.) During increase 1.5446228 1.5444540 1.5437929 intemperature (t = 55° C.)

FIG. 6 is a graph showing a chromatic aberration of magnification in themain scanning direction, which is one of effects in this embodiment(design data shown in Tables 1 to 3 are used). In FIG. 6, a differencebetween imaging positions in the main scanning direction in the casewhere a wavelength difference ΔA is set to 5 nm (λ=410 nm) with respectto a reference wavelength (λ=405 nm) is plotted. As compared with arefraction optical element composed of only refraction surfaces withoutusing the diffraction optical element 7, it is apparent that thediffraction optical element 7 has a function for sufficiently correctingthe chromatic aberration of magnification.

As described above, although the single beam semiconductor laser 1 isused as the light source means, the chromatic aberration ofmagnification is corrected. Therefore, even in an optical scanningdevice using a multi-beam light source for emitting a plurality of lightfluxes (for example, three or four light fluxes), a jitter caused in themain scanning direction is reduced, so that a preferable image qualitycan be achieved.

In the case of the optical scanning device using the multi-beam lightsource for emitting the plurality of light fluxes, it is possible toobtain an effect that a chromatic aberration of magnification in themain scanning direction due to a wavelength difference in a multi laserand a focal variation on the surface to be scanned which is caused withan increase in temperature are minimized.

FIG. 7 is a graph showing an effect obtained by only the scanningoptical system with respect to a temperature compensation in the subscanning direction, which is one of effects in this embodiment (designdata shown in Tables 1 to 3 are used). With respect to the temperaturecompensation, as disclosed in Japanese Patent Application Laid-Open No.H10-197820, a focal variation due to a temperature change of an opticalresin (reduction in refraction index of the optical resin, or the like)in the optical scanning device is corrected by a change in power of thediffraction portion resulting from a variation in wavelength of thelight source due to the temperature change. In the calculation, a changeratio of the refraction index of the used optical resin (dn/dt) is setto −7.988E-05 and a temperature characteristic of the wavelength of thelaser (dλ/dt) is set to 0.04 nm/° C. As compared with a refractionoptical element composed of only refraction surfaces without using thediffraction optical element 7, it is apparent from FIG. 7 that thediffraction optical element 7 has a sufficient temperature compensationeffect.

As described above, the diffraction order, the number of grids, theeffective range, the minimum pitch, the grid depth, and the like in thediffraction optical element are suitably set in the optical scanningdevice using the short wavelength light source having a wavelength of500 nm or less. Therefore, it is possible to provide a diffractionoptical element in which the grid shape is prevented from by beingdistorted by the mold cutting process and the mold forming process toreduce the degree of difficulty of manufacturing.

When the chromatic aberration of magnification is corrected using thediffraction optical element and the focal variation due to thetemperature change is reduced using the diffraction optical element, itis possible to provide an optical scanning device capable of obtaining apreferable image all the time.

In this embodiment, the diffraction grid is provided on the surface ofthe diffraction portion in the scanning optical system. The diffractiongrid may be provided on a plane. The diffraction grid may be provided onthe surfaces of the plurality of lenses in the scanning optical system.

Second Embodiment

FIG. 8 is an explanatory view showing an optical scanning deviceaccording to a second embodiment of the present invention, which showsan embodiment in which a diffraction grid is formed in the cylinder lens4. In this embodiment, a diffraction grid 4 a is formed on an exitsurface side of the cylinder lens 4 and has power in only the subscanning direction. The cylinder lens 4 itself is made of an opticalresin and can be produced at low cost by injection molding.

Table 4 shows curvatures of the cylinder lens 4, phase coefficients ofthe diffraction grid 4 a, and the like.

The phase function used here is the same as in the first embodiment.φ=2 πk/λ·d0·Z ² (k=2 and λ=405 nm)

In this embodiment, the diffraction grid 4 a is constructed using thesecond design order as in the first embodiment.

With respect to the diffraction grid used here, m·L·λ=111 and P/λ=56.35.Therefore, both Expression 1 and Expression 2 are satisfied. When thepresent invention is applied to the cylinder lens, there is no problemwith respect to limitations to the number of grids and the effectivewidth. With respect to the minimum grid pitch, it is necessary tocarefully design the diffraction grid as in the first embodiment.Because the grid depth is determined according to the wavelength of thelight source, the refraction index of the material, and the diffractionorder, there is no relationship with the number of grids and the gridpitch. Therefore, also in view of the grid depth, when the diffractiongrid is constructed using the secondary diffraction light, the degree ofdifficulty of manufacturing is reduced. In this embodiment as well, thegrid depth is 1.4818 (μm) as in the first embodiment, so that Expression3 is satisfied.

Table 4 shows design data in this embodiment. TABLE 4 Second ComparativeExample Embodiment (refraction system) Incident surface side R1s64.93016 24.59789 curvature radius Exit surface side R2s ∞ ∞ curvatureradius Exit surface side d0 −3.56742E−03 phase coefficient Incidentsurface to e1 6.00 6.00 Exit surface Focal distance in sub fs 45.0045.00 scanning direction Focal variation amount Δs 0 0.17356 atincreased temperature of up to 25° C. Effective width of L 5.00 5.00diffraction grid Total grid number in m 55 diffraction grid Minimumpitch of P 0.02282 diffraction grid

In this embodiment, the diffraction grid 4 a is used for temperaturecompensation in the sub scanning direction. When the cylinder lens iscomposed of only refraction surfaces, a position of a focal line nearthe polygon mirror 5 is shifted to the scanning lens side by 0.17356 mmat an increased temperature of up to 25° C. In contrast to this, thediffraction grid is used for temperature compensation in this embodimentto completely prevent the focal variation.

Third Embodiment

FIG. 9 is an explanatory view showing a part of an optical scanningdevice according to a third embodiment of the present invention, whichshows an embodiment in which a diffraction grid is formed in thecollimator lens 3. In this embodiment, a diffraction grid 3 a is formedon an incident surface side of the collimator lens 3. Because of aconcentric diffraction grid, although the single collimator lens isused, it becomes an optical system in which an on-axis chromaticaberration is corrected.

Table 5 shows curvatures of the collimator lens 3, phase coefficients ofthe diffraction grid 3 a, and the like.

The phase function used here is the same as in the first embodiment.φ=2πk/λ·c0·H ² , H ² =Y ² +Z ² (k=2 and λ=405 nm)

In this embodiment, the diffraction grid 3 a is constructed using thesecond design order as in the first and second embodiments.

With respect to the diffraction grid 3 a used here, m·L·λ=47 andP/λ=129.60. Therefore, both Expression 1 and Expression 2 are satisfied.When the present invention is applied to the collimator lens, there isno problem with respect to limitations to the number of grids and theeffective width. Even in the minimum grid pitch, the conditionalexpression is sufficiently satisfied. Because the grid depth isdetermined according to the wavelength of the light source, therefraction index of the material, and the diffraction order, there is norelationship with the number of grids and the grid pitch. Therefore,also in view of the grid depth, when the diffraction grid is constructedusing the secondary diffraction light, the degree of difficulty ofmanufacturing is reduced. In this embodiment as well, the grid depth is1.4818 (μm) as in the first and second embodiments, so that Expression 3is satisfied.

In this embodiment, the diffraction grid is used to correct the on-axischromatic aberration.

Table 5 shows design data in this embodiment. TABLE 5 Reverse tracingfrom exit surface side (parallel light side) Third Comparative ExampleEmbodiment (refraction system) Exit surface side R2 12.34417 10.9324curvature radius Incident surface side R1 ∞ ∞ curvature radius Incidentsurface side c0 −1.56392E−03 phase coefficient Exit surface to e1 3.003.00 Incident surface Focal distance f 20.00 20.00 On-axis chromatic Δm0 0.02907 aberration (Δλ = 5 nm) Effective width of L 5.00 5.00diffraction grid Total grid number in m 23 diffraction grid Minimumpitch of P 0.05249 diffraction grid

FIGS. 10A and 10B are graphs showing a spherical aberration and anon-axis chromatic aberration of the collimator lens in the case ofreverse tracing from the exit surface side (parallel light side) of thecollimator lens.

FIG. 10A shows the case where the diffraction grid 3 a is formed on theincident surface of the collimator lens. FIG. 10B shows the case of asingle lens in which the incident surface and the exit surface each area refraction surface. When the collimator lens 3 has only refractionsurfaces, an on-axis chromatic aberration of 0.029 nm is caused at awavelength difference of 5 nm. In contrast to this, the diffraction grid3 a is used to correct the chromatic aberration in this embodiment.Therefore, although the wavelength difference of 5 nm is provided, theon-axis chromatic aberration is not caused at all.

Hereinafter, a modified example will be described.

The diffraction grid may be provided for each of the collimator lens 3,the lens 7 in the scanning optical system, and the cylindrical lens 4.The diffraction grid may be provided for two of the collimator lens 3,the lens 7 in the scanning optical system, and the cylindrical lens 4.

Fourth Embodiment

FIG. 11 is a main part sectional view in the sub scanning direction,showing an image forming apparatus using the optical scanning deviceaccording to the first, second, or third embodiment. In FIG. 11,reference numeral 104 denotes an image forming apparatus. Code data Dcis inputted from an external device 117 such as a personal computer tothe image forming apparatus 104. The code data Dc is converted intoimage data (dot data) Di by a printer controller 111 in the imageforming apparatus 104. The image data Di is inputted to an opticalscanning unit 100 having the structure indicated in the first, second,or third embodiment. A light beam 103 modulated according to the imagedata Di is emitted from the optical scanning unit 100. A photosensitivesurface of a photosensitive drum 101 is scanned with the light beam 103in the main scanning direction.

The photosensitive drum 101 serving as an electrostatic latent imagebearing member (photosensitive member) is rotated clockwise by a motor115. According to the rotation, the photosensitive surface of thephotosensitive drum 101 is moved in the sub scanning directionorthogonal to the main scanning direction with respect to the light beam103. A charging roller 102 for uniformly charging the surface of thephotosensitive drum 101 is provided on an upper part of thephotosensitive drum 101 so as to be contact with the surface thereof.The surface of the photosensitive drum 101 which is charged by thecharging roller 102 is irradiated with the light beam 103 scanned by theoptical scanning unit 100.

As described earlier, the light beam 103 is modulated according to theimage data Di. The surface of the photosensitive drum 101 is irradiatedwith the light beam 103 to form an electrostatic latent image thereon.The electrostatic latent image is developed as a toner image by adeveloping device 107 provided in the downstream side from theirradiation position of the light beam 103 in the rotational directionof the photosensitive drum 101 so as to be in contact with thephotosensitive drum 101.

The toner image developed by the developing device 107 is transferredonto a sheet 112 serving as a transfer material by a transfer roller 108provided below the photosensitive drum 101 so as to oppose thephotosensitive drum 101. The sheet 112 is contained in a sheet cassette109 located in the front (right side in FIG. 11) of the photosensitivedrum 101. Manual sheet feeding is also possible. A feed roller 110provided at the end portion of the sheet cassette 109 serves to feed thesheet 112 in the sheet cassette 109 to a transport path.

By the above operation, the sheet 112 to which an unfixed toner image istransferred is further transported to a fixing device located in therear (left side in FIG. 11) of the photosensitive drum 101. The fixingdevice is composed of a fixing roller 113 having a fixing heater (notshown) therein and a pressure roller 114 provided so as to press thefixing roller 113. The sheet 112 transported from a transferring part isheated while it is pressurized by a press-contacting part between thefixing roller 113 and the pressure roller 114, so that the unfixed tonerimage on the sheet 112 is fixed. Further, a delivery roller 116 isprovided in the rear of the fixing roller 113 and the fixed sheet 112 isdelivered to the outside of the image forming apparatus 104 by thedelivery roller 116.

Although not shown in FIG. 11, the printer controller 111 conducts notonly data conversion described earlier but also control of each part ofthe image forming apparatus 104, which is represented by the motor 115,control of a polygon motor in the optical scanning unit as describedlater, and the like.

FIG. 12 is a main part schematic diagram showing a color image formingapparatus according to the first, second, or third embodiment of thepresent invention. This is a tandem type color image forming apparatusin which four optical scanning devices are arranged to record imageinformation in parallel on the surfaces of the photosensitive drumsserving as the image bearing members. In FIG. 12, reference numeral 60denotes a color image forming apparatus; 11, 12, 13, and 14 each denotethe optical scanning device having the structure described in the firstembodiment; 21, 22, 23, and 24 each denote the photosensitive drumsserving as the image bearing members; 31, 32, 33, and 34 each denote adeveloping device, and 51 denotes a transport belt.

In FIG. 12, respective color signals of R (red), G (green), and B (blue)are inputted from an external device 52 such as a personal computer tothe color image forming apparatus 60. The color signals are convertedinto respective image data (dot data) of C (cyan), M (magenta), Y(yellow), and B (black) by a printer controller 53 in the color imageforming apparatus 60. These image data are separately inputted to theoptical scanning devices 11, 12, 13, and 14. Light beams 41, 42, 43, and44 modulated according to the respective image data are emitted from theoptical scanning devices 11, 12, 13, and 14. The photosensitive surfacesof the photosensitive drums 21, 22, 23 and 24 are scanned with the lightbeams in the main scanning direction.

With the color image forming apparatus according to this embodiment, thefour optical scanning devices (11, 12, 13, and 14) are arranged, eachdevice corresponding to C (cyan), M (magenta), Y (yellow), and B(black), and the image signals (image information) are recorded inparallel on the surfaces of the photosensitive drums 21, 22, 23 and 24by the optical scanning devices, thus printing a color image at highspeed.

With the color image forming apparatus according to this embodiment, asdescribed above, the latent images of the respective colors are formedon the corresponding surfaces of the photosensitive drums 21, 22, 23 and24 using the light beams based on the respective image data from thefour scanning optical devices 11, 12, 13, and 14. After that, themulti-transfer is conducted on a recording material to produce a fullcolor image.

For example, a color image reading apparatus including a CCD sensor maybe used as the external device 52. In this case, the color image readingapparatus and the color image forming apparatus 60 compose a colordigital copying machine.

According to the present invention, in the optical scanning devicehaving the diffraction grid, for which the short wavelength light sourcehaving a wavelength of 500 nm or less is used, the grid shape can beeasily produced by selecting a suitable design order of the diffractiongrid.

This application claims priority from Japanese Patent Application No.2003-208957 filed on Aug. 27, 2003, which is hereby incorporated byreference herein.

1. An optical scanning device, comprising: light source means foremitting at least one light flux having a single wavelength of 500 nm orless; deflecting means for deflecting the at least one light fluxemitted from the light source means; a first optical system for guidingthe light flux emitted from the light source means to the deflectingmeans; and a second optical system for imaging the light flux deflectedby the deflecting means on a surface to be scanned, wherein one of thefirst optical system and the second optical system includes adiffraction grating, and wherein a design order of the diffractiongrating is the second order to the fifth order. 2-16. (canceled)
 17. Animage forming apparatus, comprising: the optical scanning deviceaccording to claim 1; a photosensitive member that is located on thesurface to be scanned; a developing device for developing, as a tonerimage, an electrostatic latent image that is formed on thephotosensitive member scanned with the light flux by the opticalscanning device; a transfer device for transferring the developed tonerimage to a transfer material; and a fixing device for fixing thetransferred toner image to the transfer material.
 18. An image formingapparatus, comprising: the optical scanning device according to claim 1;and a printer controller that converts code data inputted from anexternal device into an image signal and outputs the image signal to theoptical scanning device.
 19. A color image forming apparatus,comprising: a plurality of optical scanning devices, each of which is anoptical scanning device according to claim 1; and a plurality of imagebearing members for forming images having colors different from oneanother, each of which is located on a surface to be scanned in arespective one of the plurality of optical scanning devices.
 20. A colorimage forming apparatus according to claim 19, further comprising aprinter controller that converts a color signal inputted from anexternal device into image data in different colors and outputs theimage data to the plurality of optical scanning devices.